In the circulation of most, if not all, rhesus monkeys (Macaca mulatta) there is a substance which bears a physicochemical as well as a limited immunological resemblance to rhesus monkey luteinizing hormone (rhLH). A second LH-like substance (LHS), immunologically similar to the serum LHS but of greater molecular size, is present in rhesus monkey pituitary glands. The objectives of the proposed research are: 1) to determine the biological activity of the serum LHS, and to investigate the source, mode, and control of secretion of the serum LHS in selected female monkeys through measurement of LHS blood levels before and after chronic estrogen treatment and hypophysectomy, relying on differential activities in two rhLH radioimmunoassays to distinguish LHS from rhLH; 2) to characterize the pituitary LHS after purification by chromatographic, electrophoretic, and immunochemical means; and 3) to explore the possibility of a relationship between pituitary LHS and either serum LHS or rhLH by attempted conversion, both in vivo and in vitro, of pituitary LHS to serum LHS or rhLH.